This invention relates to apparatus for the production of electrons and more particularly to such apparatus employing a laser energized thermionic process to generate high density pulses of electrons.
Pulsed sources of electrons of high current density are of value for many purposes, such as for welding and metal working devices, high resolution electron microscopy, intense short burst x-ray emitters, high current switches, electron beam systems, microwave generators and free electron lasers. It is well known to generate pulses of electrons thermionically, i.e., by heating to a high temperature a suitable metallic cathode with an intense laser beam to drive off electrons. The currents so generated, however, have been limited generally to tens of amperes. Furthermore, these electron emissions were achieved typically with focused incident beams which caused target surface damage through vaporization and ionization and hence were not purely thermionic processes. In other known attempts to create high density pulses of electrons the laser flux densities were kept below the destruction level of the metallic target. However, in such cases, very low peak current densities were achieved.
It is, therefore, an object of this invention to generate thermionically pulses of electrons of reasonable spatial area and high current density, for example, current densities of four to six kiloamperes per square centimeter and currents in the range of two to three kiloamperes with pulse widths varying from nanoseconds to tens of microseconds.
It is a further object of the invention to produce such pulses at kilohertz pulse repetition rates.
Yet another object is to provide apparatus in which the electron emitting surface is not damaged by the incident laser flux. Still other objects of this invention are to provide a fast acting, laser driven switch and a laser heated thermal-to-electrical energy converter using the thermionic process. Other objects and advantages of the invention will be apparent in what follows.